1. Field of the Invention
The present invention relates to a high frequency amplifier, and in particular to a linear gain control amplifier capable of implementing a gain linearity and a wide variable gain range.
2. Description of the Background Art
In a communication system, an input RF (radio frequency) signal is amplified by two amplifiers and is outputted as an output RF signal. In this case, the input RF signal is a signal having a wide gain range in the view of a receiver. In addition, in view of a transmitter, the input RF signal is a signal containing information to be transmitted. In view of the receiver, the output RF signal is a signal containing information used for the next circuit block. In view of the transmitter, the output RF signal is a signal having a wide gain range and is a signal to be amplified for transmitting the input RF signal over a certain distance.
Therefore, in the field of mobile communication systems, there is a need for a variable gain amplifier that includes a wide gain range for constantly receiving signals of various strength and for transmitting signals with signal strength based on a corresponding distance. Here, the variable amplifier should be provided with a characteristic that the gain is linearly varied based on a control voltage.
Generally, a variable gain amplifier is formed of a MOSFET, and the gain of the variable gain amplifier has a non-linear characteristic with respect to the control voltage. The reason why the gain has a non-linear characteristic is that the control voltage linearly controls the output voltage corresponding to the input voltage. The gain is a log value of the output signal, and thus the gain has a log characteristic with respect to the control voltage.
FIG. 1 illustrates the elements of a conventional linear gain control amplifier. The conventional linear gain control amplifier includes first and second amplifiers 12 and 16 which are formed in a two-stage structure to obtain a wide variable gain range. The first and second amplifiers 12 and 16 are each a variable gain amplifier formed of a dual gate MOSFET and their gain has a non-linear characteristic with respect to a control voltage. The conventional linear gain control amplifier also includes a compensation circuit 10 so that the control voltages of the first and second amplifiers 12 and 16 has an exponential function characteristic.
The compensation circuit 10 compensates a gain control signal AGCin outputted from an external loop control output (not shown) and outputs a compensation voltage for compensating the non-linearity of the first and second amplifiers 12 and 16. Here, the compensation signal has an exponential function characteristic. If the compensation signal has an exponential function characteristic, the gain characteristics of the first and second amplifiers 12 and 16 has a linearity.
FIG. 2 is a circuit diagram illustrating the compensation circuit 10. The inverting input terminal of an operational amplifier OP receives the gain control signal AGCin through a resistor R1 and is connected with an output terminal of the operational amplifier OP via a resistor R2 connected in parallel with a variable resistor R3 and in series with a diode D1. In addition, the non-inverting input terminal of the operational amplifier OP is connected with a supply voltage (-VCC) terminal via a resistor R4 and is connected with the ground voltage VSS via a reverse biased diode. Here, the resistors R1, R2 and R4 have the same resistance value of e.g. 10 K.OMEGA., and the variable resistor R3 has a resistance value of e.g. 50 K.OMEGA.. In addition, the diodes D1 and D2 are of the same type for implementing a feed back operation with respect to the temperature variation. The diode D2 may be substituted by a device such as a thermistor.
The filter 14 is formed of a Surface Acoustic Wave (SAW) filter having a band pass filtering function, and filters an undesired frequency component from an output of the first amplifier 12.
The operation of the conventional linear gain control amplifier will be explained with reference to the accompanying drawings. When a gain control signal AGCin is inputted from an external loop control output (not shown), the compensation circuit 10 compensates the gain control signal AGCin and outputs a compensation signal having an exponential function characteristic to the first and second amplifiers 12 and 16, respectively.
In the compensation circuit, as shown in FIG. 2, if the gain control signal AGCin is larger than a reference voltage Vref at the non-inverting terminal of the operational amplifier OP (AGCin&gt;Vref), a first slope of the compensation signal nearly becomes (R3.parallel.R2)/R1. If the gain control signal AGCin is smaller than the reference voltage Vref(AGCin&lt;Vref), a second slope of the compensation signal nearly becomes R3.parallel.R1. Here, the second slope of the compensation signal is greater than the first slope. As a result, the compensation signal having the first and second slopes is outputted by the operational amplifier OP, which operates as an adder, thereby exponentially approximating the compensation signal to the exponential function.
Therefore, the first and second amplifiers 12 and 16 amplify the input signal RFin at the gain level determined by the compensation signal and the linear gain amplifier outputs an output signal Rfout. Thus, the gains of the first and second amplifiers 12 and 16 have a linearity based on the compensation signal having the exponential function characteristic. The filter 14, namely, the SAW filter having a band pass characteristic, filters the output of the first amplifier 12 and removes an undesired frequency component. However, since the insertion of the SAW filter causes an insertion loss (by about 20dB), the output of the first amplifier 12 and the input of the filter 14 are impedance-matched to minimize the insertion loss.
As described above, the conventional linear gain control amplifier uses the first and second amplifiers 12 and 16 to obtain a wide variable gain range. The conventional linear gain control amplifier also uses the compensation circuit 10 to compensate the gain control signal AGCin exponentially. Thus, the gain characteristic of the first and second amplifiers 12 and 16 has a linearity with respect to the control voltage.
However, the conventional linear gain control amplifier has the following disadvantages. First, since the conventional compensation circuit uses two power supply voltages +VCC and -VCC, the construction of the system becomes complicated.
Second, a diode may be implemented in a bipolar fabrication process, but a diode may not be easily implemented in a standard CMOS fabrication process. An additional process must be used to implement the diode in the standard CMOS process. Thus, since the compensation circuit has the diodes, the compensation circuit may not be easily implemented by the standard CMOS process.
Third, the SAW filter may not be integrated into an integrated circuit IC, and the compensation circuit also may not be implemented as one integrated circuit IC.